Crystal and molecular structure of 4-fluoro-1H-pyrazole at 150 K

Two crystallographically unique 4-fluoro-1H-pyrazole moieties linked by an N—H⋯N hydrogen bond are found in the asymmetric unit. Unlike the trimeric supramolecular motifs found in the structures of the chloro and bromo analogues, 4-fluoro-1H-pyrazole forms one-dimensional chains by intermolecular hydrogen bonding in the crystal.

Only two 4-halo-1H-pyrazole crystal structures are known to date (chloro and bromo, the structure of 4-iodo-1H-pyrazole has not been reported yet). The triclinic structure of 4-fluoro-1H-pyrazole, C 3 H 3 FN 2 (P1), reported here is not isomorphous with those of the chloro and bromo analogues (which are isomorphous, orthorhombic Pnma). To avoid sublimation during the measurement, diffraction data were collected at 150 K. Two crystallographically unique 4-fluoro-1H-pyrazole moieties linked by an N-HÁ Á ÁN hydrogen bond are found in the asymmetric unit. Unlike the trimeric supramolecular motifs found in the structures of the chloro and bromo analogues, 4-fluoro-1H-pyrazole forms onedimensional chains by intermolecular hydrogen bonding in the crystal.

Chemical context
1H-Pyrazole (pzH) is both a hydrogen-bond donor and acceptor molecule, owing to its NH and N centers. Consequently, pyrazole moieties of the parent compound or Csubstituted analogues form hydrogen bonds to each other in the corresponding crystal structures, and similarly to imidazole, have higher melting and boiling points than other fivemembered cyclic aromatic molecules lacking either the hydrogen-bond acceptor (pyrrole), the hydrogen-bond donor (N-methyl derivatives, furan, isoxazole, oxazole, thiophene, isothiazole, thiazole) or both centers (cyclopentadiene) (Fig. 1). The proximity of the hydrogen-bond donor and acceptor centers in pz allows for the formation of either Comparison of the structures of five-membered aromatic heterocycles and their corresponding melting and boiling points ( C) according to the CRC Handbook of Chemistry and Physics (Rumble, 2022) or the literature. Those with no melting points reported are liquids at room temperature. discreet hydrogen-bonded motifs, such as dimers, trimers, tetramers and hexamers, or polymeric catemers depending on the substituents (Bertolasi et al., 1999;Foces-Foces et al., 2000;Claramunt et al., 2006;Alkorta et al., 2006), whereas imidazole only forms catemers (Cammers & Parkin, 2004).
The two crystallographically independent 4-Fpz moieties, which are identical within experimental error, are planar with deviations from the C 3 FN 2 mean-plane of less than 0.004 and 0.008 Å , respectively. Table 1 presents a comparison of bond lengths determined by X-ray diffraction for the parent pzH at 150 K (Sikora & Katrusiak, 2013), 4-FpzH at 150 K, 4-ClpzH at 150 K (Rue & Raptis, 2021) and 4-BrpzH at room temperature (Foces-Foces et al., 1999). All structures contain two symmetry-independent moieties. In the case of pzH and 4-FpzH, the NH and N centers of the pz rings are distinct, unlike in the case of 4-Cl/BrpzH. Therefore, in the former case two distinct sets of C-N and C-C bond distances are observed. Similarly to pzH, in 4-FpzH the C-N bond adjacent to N is shorter than the one adjacent to NH [by 0.008 (1)/ 0.010 (1) Å ], whereas the C-C bond adjacent to N is longer than the one adjacent to NH [by 0.019 (1)/0.018 (1) Å ]. In general, the N-N, C-N and C-C bond distances in 4-RpH are consistent across the R = H, F, Cl and Br series.

Supramolecular features
The pz N-H proton donates an N-HÁ Á ÁN hydrogen bond to a neighbouring pz unit on one side, while the pz N atom accepts an N-HÁ Á ÁN hydrogen bond from another pz unit on the opposite side (Table 2 Displacement ellipsoid plot (50% probability) of the crystal structure of 4-fluoro-1H-pyrazole, showing the contents of the asymmetric unit. Table 1 Comparison of N-N, C-N and C-C bond lengths determined by X-ray diffraction in 4-RpzH (R = H, F, Cl and Br).
Numbers marked with an asterisk indicate average values in the case of pyrazoles with disordered NH/N centers.

Figure 5
Packing diagram (along the c axis) of 4-fluoro-1H-pyrazole, showing the herringbone-type packing of the pyrazole moieties.

Figure 6
Comparison of the hydrogen-bonded motifs (two different views for each) in the crystal packing of 4-RpzH (R = H, F, Cl and Br).

Synthesis and crystallization
4-Fluoro-1H-pyrazole was synthesized according to a published procedure, from sodium fluoroacetate (WARNING: highly toxic!) by reaction with oxalyl chloride and dimethylformamide, followed by treatment with base and then hydrazine (England et al., 2010). Single crystals were obtained from the powder by slow isothermal sublimation inside a sealed vial under ambient conditions.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.